The invention relates to a control system for an automatic transmission as may be used in an automotive vehicle, and in particular, to a control system for an automatic transmission in which the occurrence of a creep phenomenon is prevented when a load (a car body) is at rest and a driver is not willing to start the vehicle.
In the general use of an automatic transmission for an automotive vehicle, for example, the vehicle will free to move when a manual shift lever is turned to a position corresponding to a range which permits a running of the vehicle, for example, in a drive (D) range if the vehicle is at rest and the driver is not willing to start the vehicle or when an accelerator pedal assumes its off position unless a braking operation is applied intentionally. This is caused by a drive torque appearing at the output shaft of the transmission during the idling mode of an engine which will be greater than necessary inasmuch as the speed reduction ratio of the transmission is set at its maximum (to select a first speed range) when the vehicle is at rest, and is commonly referred to as a creep phenomenon. When the vehicle is subject to a creep phenomenon, it is necessary to apply the brake and maintain it operative as long as the vehicle remains at rest, forcing a driver to perform a troublesome operation. To accommodate for this, there is disclosed a technique in Japanese Laid-Open Patent Application No. 83,526/1973 in which the occurrence of a creep phenomenon is prevented by automatically shifting to a second or a higher speed range whenever the vehicle is stopped, thereby reducing the reduction ratio to reduce the magnitude of the output torque.
In the conventional practice, the occurrence of a creep phenomenon is prevented as follows: Specifically, when a vehicle comes to a stop and a throttle opening is reduced to a given value or less (corresponding to an off condition of an accelerator pedal), it is determined that a driver is not willing to start the vehicle, and a shift occurs from a first to a second (or a third) speed range. Subsequently, when the throttle opening increases to a given value (corresponding to an on condition of the accelerator pedal), it is determined that the driver is willing to start the vehicle, and a shift is made automatically from the second to the first speed range, thereby increasing the drive torque upon starting the vehicle.
Such control presents no problem whatsoever in a normal operation. However, it is found that a problem arises when a particular operation is performed. Thus, if a brake pedal is depressed while depressing the accelerator pedal when the vehicle is at rest, and the accelerator pedal is then returned without allowing the vehicle to run, an associated control system will shift from the second to the first speed range immediately after the depression of the accelerator pedal, and will shift from the first to the second speed range immediately upon returning the accelerator pedal. In such instance, a special double engagement occurs within the transmission, the detail of which will be described later, thereby giving rise to oscillations of the output shaft of the transmission. This is inevitable in an automatic transmission of a general construction which employs a one way clutch.